Reproductive Dynamics of the Seagrass Thalassia Hemprichii (Ehrenberg) Ascherson in Iligan Bay Roseller G. Sabilla and Wilfredo
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Reproductive Dynamics of the Seagrass Thalassia hemprichii (Ehrenberg) Ascherson in Iligan Bay Roseller G. Sabilla and Wilfredo H. Uy Mindanao State University at Naawan, 9023 Naawan, Misamis Oriental, Philippines [email protected] ABSTRACT A study on the reproductive dynamics of the dugong grass, Thalassia hemprichii (Ehrenberg) Ascherson was carried out on the seagrass meadows of Tubajon, Laguindingan and Poblacion, Naawan, Misamis Oriental, and Danlugan, Lopez Jaena, Misamis Occidental from February to May, 2005. Age reconstruction technique through the analysis of leaf scars along the vertical rhizomes indicated that the flowering frequency of T. hemprichii is once every two years. The first flowering occurs between 1.5 to 2 years based on the presence of flower scars on vertical rhizomes. The average leaf plastochrone interval (LPI) on the shoots of T. hemprichii across sites was 6.4 days. The estimated biomass allocated to flowering ranged from 0.5 to 0.72 g DW m-2. Spatial differences in the morphology of flowers and fruits of T. hemprichii were observed across stations in the three study sites. The average number of fruits during peak reproductive season ranged from 5.5 to 6.2 m-2, with an average number of 2.0 to 2.6 seeds fruit- 1. These results indicate that the sexual reproduction of T. hemprichii is active and its capacity to provide new recruits from seedlings is relatively high. Key words: Age reconstruction, flowering frequency, leaf plastochrone interval, recruits. INTRODUCTION Seagrasses are submerged marine angiosperms found mostly in all coastal waters of the world except in the Antarctic (Den Hartog, 1970). About 60 species of seagrasses have been recorded worldwide (Kuo and McComb, 1989; Den Hartog, 1970), 16 species of which are reported to occur in the Philippines. Seagrass beds are recognized world-wide for their economic and ecological importance. They help reduce current and wave energy, filter suspended sediments from water, and stabilize bottom sediments (Fonseca and Cahalan, 1992; Fonseca, et al. 1982) and serve as primary producers, habitats, shelters and food for fishes and marine invertebrates, many of which are economically important. Seagrass ecosystems in the ASEAN region, however, are threatened by both natural and human-induced disturbances and their physical loss is at ever increasing rates (Short, et al., 1988; Short and Wyllie-Echeverria, 1996; Rollon, 1998). Along Philippine coasts seagrass losses are largely due to destructive fishing methods and increasing pollution and siltation (Fortes, 1990). Seagrasses have been studied quite extensively in several bio-geographical regions, particularly in temperate zones (Den Hartog, 1970; Hemminga and Duarte, 2000). Reproductive Sabilla & Uy: Reproductive dynamics of the seagrass… 29 dynamics of seagrasses in tropical areas have been rarely investigated, particularly on the aspects of (1) the reproductive capacity, (2) fruit and seed production potential, (3) age at first maturity and (4) flowering frequency. These important aspects of seagrass dynamics have been studied by earlier researchers in the Caribbean region (Tomasko and Lapointe, 1991; Gallegos, et al., 1993; Tussenbroek, 1996) and Spanish Mediteranian (Marba, et al., 1994). Related studies in the tropics were conducted in Kenya (Duarte et al., 1996), Indonesia (Erftmeijer, 1993) Bolinao, Pangasinan, Philippines (Vermaat, et al., 1995; Duarte, et al., 1994) and Negros in Central Philippines (Biyo, et al., 2001). The present study aims to elucidate the reproductive dynamics of the dugong grass, Thalassia hemprichii, in selected sites of Iligan Bay in terms of flowering intensity, flowering frequency, fruit and shoot morphology, and age at first maturity. These are vital information that would be useful in developing management strategies for conservation of seagrass resources in the country. MATERIALS AND METHODS The study was conducted from February to May 2005 in the seagrass meadows of the coastal municipalities of Laguindingan and Naawan, Misamis Oriental and Lopez Jaena, Misamis Occidental (Fig. 1). The areal extent of the seagrass meadows in every station was determined using a Global Positioning System (GPS). Three stations in every site were established to represent the shallow, exposed areas during low tide (S1); submerged seagrass beds with depth of 30 cm (S2); and the deeper parts with depths greater than 60 cm (S3). Figure 1. Map of Iligan bay showing the location of the three study sites. 30 J. Environmental Science and Environment, Vol. 3 (2015) Flowering shoot density Sampling to determine shoot density of flowering T. hemprichii was conducted during the peak reproductive period (based on monthly observations made from 2003 to 2005) on February 2005 in all sites while collection of mature fruits was done in May 2005. Sampling was done by throwing quadrats (0.5 x 0.5m) randomly 20 times in the seagrass meadow in every station during low tide. The shoot density of the T. hemprichii was estimated by counting the number of standing shoots within the 0.25 m2 quadrat. All shoots of T. hemprichii bearing flowers and fruits found inside the quadrat were collected, placed in plastic bags with appropriate labels, and brought to the MSU at Naawan research laboratory for morphological analysis and biomass estimation. Shoot morphology and flower biomass In the laboratory, all harvested male and female flowering shoot samples were cleaned carefully by removing the sediments and other debris. After washing in tap water, samples were wrapped with aluminum foil, labeled, placed in an aluminum tray and oven-dried for 48 h at 50- 600C. Desiccation process after oven-drying followed within two to three days to get the constant weight expressed as in g DW m-2. Morphological attributes (i.e., age, shoot height, stem height, leaf width, number of leaves, flower peduncle height, number of flower scars) of the flower-bearing shoots were determined and compared across stations and study sites. The flowers, oven-dried to constant weight (DW), were held in a porcelain crucible and placed in a muffle furnace (Barnstead Thermolyne) set at 555oC for combustion for about five hours, afterwards, the remaining material in the crucible was weighed in a Sauter analytical balance. The ash free dry weight (AFDW) of each sample was then obtained by subtracting the weight of remaining material from the DW of the sample (Kendrick and Lavery, 2001). Morphology and biomass of fruits and seeds In the laboratory, the morphology of T. hemprichii shoots where the fruits were attached was characterized by measuring the shoot height, stem height, leaf width, number of leaves, number of flower scars, number of fruits and presence of new flowers. The age of each shoot was estimated by counting the number of leaf scars then multiplying this by the leaf plastochrone interval (LPI). The fruits T. hemprichii were washed and cleaned by removing the sediments and other attached particles then all the seeds and pericarp of the fruits were weighed using a digital weighing scale. The length and width of seeds were measured to the nearest mm using a vernier caliper. Fruit morphology parameters (i.e., fruit width, fruit height, stylar beak length, fruit peduncle height, number of seeds) were carefully measured and recorded to establish comparative differences across stations. All the seeds of each capsule or pericarp were separated then their sizes (i.e. height and width) were measured using a vernier caliper. Wet weight of each seed was taken using a digital weighing scale and then oven-dried for at least 48 h at 50-60oC to constant weight. Sabilla & Uy: Reproductive dynamics of the seagrass… 31 Leaf plastochrone interval (LPI) determination Leaf plastochrone interval is the period of time needed to produce two successive leaves. The LPI of T. hemprichii was determined by leaf marking technique described in Uy (2001) continuously from August 5 to September 10, 2004. A total of 20 shoots occurring at different depths in the three study sites were marked by tying a colored ribbon around the stem of each shoot. The end of the youngest leaf of each marked shoot was clipped so that the new leaves formed during the observation period could be identified. The marked shoots were collected after 1-2 weeks, and the number of new leaves produced during the marking period was determined. Leaf plastochrone interval (LPI) was determined using the following equation (Duarte, et al., 1994; Vermaat, et al., 1995): Shoot age reconstruction Ten shoots of T. hemprichii with long vertical rhizomes were collected from each site The growth history of each rhizome was reconstructed from the scars left by abscised leaves and flowers on the long-lived rhizome or the seasonal signal imprinted in the flowering and size of their modules. Age reconstruction of all the shoots was determined with the aid of a dissecting microscope and a magnifying lens. Shoot age was estimated based on the number of standing leaves plus the number of leaf scars on the shoots. The inverse of the number of internodes in a cycle (number of leaves yr-1) corresponds to the annual average leaf plastochrone interval and was used to convert LPI values into absolute time units (days) Determination of age at sexual maturity and flowering frequency The age at sexual maturity of T. hemprichii shoot, or its age at the first time of flowering, was determined from the shoot samples used in age reconstruction and from shoots bearing flowers which were collected from sampled quadrats in the study sites. The age of a shoot at flowering is determined from the number of leaf scars from the peduncle scar up to the insertion of the shoot on the horizontal rhizome (Duarte, et al., 1997). With known LPI values converted into the number of days, age at sexual maturity was determined by counting the nodes from the insertion of the vertical stem into the horizontal rhizome up to the first flower scar.